CD8 expressing T cells (T/CD8+ ) play a critical role in eradicating intracellular parasites such as viruses. T/CD8+ recognize MHC class I molecules in a complex with peptides of 8 to 10 residues derived from viral proteins located in the cytosol. There is tremendous interest in the mechanism by which peptides are generated. Indirect evidence implicates proteasomes in the generation of antigenic peptides. Proteasomes are abundant macromolecular structures present in the cytosol and nucleus, and have multiple protease activities. They are thought to be responsible for energy dependent proteolysis in which ubiquitin plays a prominent role in targeting proteins for destruction. In this project we examine the contribution of proteasomes to the production of antigenic peptides and also explore the nature of substrates acted on by cytosolic peptidogenic proteases. Another goal is to identify sequences that enhance the immunogenicity of proteins by increasing their capacity to generate antigenic peptides.